1. Field of the Invention
The present invention relates to an insert type cutting instrument including an instrument main body in which an insert seat is formed and an insert having a cutting edge detachably attached to the insert seat, the insert, and a method for attaching the insert to the instrument main body.
2. Description of the Related Art
As an insert type cutting instrument, the insert type cutting instrument including a circular-cylindrical instrument main body and an insert is well known. In the insert type cutting instrument, the instrument main body has a tip pocket formed on an outer side surface of a head portion of the instrument main body, and an insert seat located backward of the tip pocket in a reverse rotation direction of the instrument main body. The insert having a cutting edge is attached to the insert seat.
The insert as mentioned above is formed like a polygonal flat plate. A through-hole going through the insert in a thickness direction of the insert is formed in the insert. A clamping screw is inserted into the through-hole, the clamping screw is tightly screwed into a screw hole formed in the insert seat, and thereby the insert is tightly attached to the insert seat.
Recently, in order to improve the efficiency of cutting, there is a tendency to speed up the rotation speed of the instrument main body. In particular, in cutting of an aluminum or aluminum base alloy, the instrument main body is rotated at a speed over 20,000 rpm. When the instrument main body is high-speed rotated as mentioned above, a centrifugal force acts on the insert with increasing rotation speed of the instrument main body, and thereby the insert may be displaced with respect to the instrument main body. If the insert is displaced, it is impossible to perform cutting with high accuracy, and the insert may become damaged due to cutting resistance.
In Published Japanese Translation No. 2000-504280 of the PCT International Application, a cutting instrument having an instrument main body and inserts is proposed. Each of the inserts is tightly attached to the instrument main body by two clamping screws. In the insert type cutting instrument, the insert is formed like a rectangular flat plate. The insert is located so that long side edges of the insert are substantially parallel to an axis of the instrument main body, and then the insert is tightly attached to the instrument main body using the clamping screw and another clamping screw located closer to a base end of the instrument main body than the clamping screw.
In the insert type cutting instrument in Published Japanese Translation No. 2000-504280 of the PCT International Application, the clamping screw located close to a tip of the instrument main body descends toward the axis of the instrument main body. In addition, another clamping screw located close to the base end of the instrument main body descends toward the axis of the instrument main body and descends toward the base end of the instrument main body. That is, a part of the insert located so as to be close to the tip of the instrument main body is pushed toward the axis of the instrument main body (in a direction orthogonal to the axis of the instrument main body), and a part of the insert located so as to be close to the base end of the instrument main body is pushed toward the axis of the instrument main body, and is pushed toward the base end of the instrument main body (in a direction obliquely intersecting with the axis of the instrument main body).
When the centrifugal force acts on the insert by rotating the instrument main body, the insert tends to separate from the axis of the instrument main body along a wall surface of an insert seat facing to the tip of the instrument main body. Further, a moment acts on the insert centering around a corner of the insert located near the wall surface of the insert seat close to an outer circumferential side end portion of the instrument main body by the centrifugal force acting on a center of gravity of the insert. If the part of the insert close to the tip of the instrument main body is heavy, the moment increases extremely.
In the insert type cutting instrument in Published Japanese Translation No. 2000-504280 of the PCT International Application, the insert is pushed in a direction obliquely intersecting with the centrifugal force, and is pushed in a direction obliquely intersecting with the moment, by the clamping screw located close to the tip of the instrument main body. Therefore, the insert may be not tightly attached to the instrument main body.
Further, a positional relationship between the two through-holes may get out of imperceptibly order due to a dispersion of the accuracy with cutting the two through-holes in the insert. If the distance between the two through-holes is shorter than its order, the insert is pushed to the tip of the instrument main body by the clamping screw located close to the tip of the instrument main body. Accordingly, the two clamping screws push the insert in conflicting directions to each other, and thereby the insert can not be not tightly attached to the instrument main body. As a result, a cutting accuracy by the insert type cutting instrument may deteriorate by a displacement of the insert caused by the centrifugal force, and the insert may become damaged.
The present invention is conceived in view of the above-described circumstances, and it is an object thereof to provide an insert type cutting instrument which enables tight attachment of an insert so as not to move by the centrifugal force while the instrument main body is high-speed rotated, the insert included in this insert type cutting instrument, and a method for attaching this insert to an instrument main body.